Transmission diversity refers to a situation where at least two transmitters are transmitting the same data flow to a receiver. For example, a mobile station may receive radio transmissions from two base stations of a cellular network simultaneously. One of the advantages of the transmission diversity is that if the quality or strength of one of the received radio transmissions decreases, the quality of some other received transmissions may still be high enough for receiving data reliably.
A situation where a mobile station communicates with more than one base stations simultaneously is called a soft handover. If necessary, for example to ensure a reliable data transmission, a mobile station may be communicating even most of the time with more than one base stations. In certain situations, when the signal of a certain base station is clearly stronger that the signals of other base stations, it may be enough to communicate with one base station only. There has to be a method for deciding with which base stations a mobile station communicates.
Transmission diversity and soft handovers can be supported, for example, in cellular networks that employ Code Division Multiple Access (CDMA) methods. Each transmitter may use its own spreading code in the downlink transmission, and a mobile station wanting to receive data from many transmitters processes the radio signal it receives with the spreading codes corresponding to the transmitters. The code sequences of different connections must be chosen so that they do not correlate and the code sequence of a specific connection has to autocorrelate. Those signals that have been spread using a code sequence that correlates with the code sequence used in receiving the seemingly white noise radio transmission are separated. The receiver has to know the transmitters code sequence and the code sequences must be synchronized. The bits in the code sequence are called chips.
FIG. 1 presents a schematic drawing of a cellular network which comprises base stations 101a, 101b and 101c. In FIG. 1, these base stations are all connected to a single radio network controller 102 and each base station is in the middle of a cell. FIG. 1 presents one mobile station 10. The base stations transmit downlink data to mobile stations (arrows 120a, 120b and 120c) and receive uplink data from mobile stations (arrows 121a, 121b and 121c).
In general a base station may comprise many transmitters, each of which transmit a separate radio signal. In systems employing CDMA methods, the transmitters may use different spreading codes. Here term cell is used to refer either to a base station or, if a base station comprises many transmitters, to a transmitter. In a situation where a mobile station receives good quality downlink transmissions from many cells, it has to be decided which cell a certain mobile station communicates with.
Usually the cellular network informs the mobile station of the possible cells, for example, selected based on the location of the mobile station. The information about the nearest cells is called neighbor list. In a cellular network which employs CDMA methods, the neighbor list may comprise the downlink spreading codes of the cells. By taking the spreading codes listed in the neighbor list into use, the mobile station may separate the data flows sent to it from each cell from the radio signal it receives. The neighbor list of the mobile station 110 may comprise, for example, the cells corresponding to base stations 101a, 101b and 101c, assuming that a base station corresponds to one cell.
Usually a pilot signal is transmitted in each cell. This pilot signal carries no changing data, so it can be quite straightforwardly used in estimating the quality of the downlink radio transmission of a certain cell. A mobile station may, for example, estimate the quality of the radio transmission of all the cells in the neighbor list. A suitable parameter for quality estimation is, for example in a cellular system employing CDMA methods, the EC/I0 ratio, where EC is energy per chip and I0 is the interference. Any other parameter measuring the quality of the signal may also be used.
The cells with which a mobile station communicates form the active set of that mobile station. A radio network controller, for example, directs the downlink data heading to a certain mobile station, to all the cells in the active set. Correspondingly, the mobile station listens to the downlink transmissions of all the cells in the active set. For example, the cells corresponding to base stations 101a and 101b can form the active set of the mobile station 110.
When the mobile station changes its location or the qualities of the downlink radio transmissions of the neighboring cells change for some other reason, it may be necessary to modify the active set. A cell may be added to or removed from the active set, or a cell in the active set may be replaced with another cell. This replacement is usually called branch replacement.
There has to be a criterion for accepting a new cell for the active set. The CDMA2000 RTT description, for example, defines the following criterion for a test cell to be accepted to the active set. The cell may then be added to the active set or, in case of branch replacement, it may replace the worst quality cell in the active set. If the quality factor, for example the EC/I0 ratio, is marked with Pi for each cell i in the active set, an acceptance limit Q can be calculated byQ=max{S·10 log10(ΣPi)+A,T}where S, A and T are parameters. As can be seen from the formula for Q, the value of Q is expressed in dB and the quality factor Pi is a plain number. It is checked if a certain cell with quality factor PT is a proper candidate for the active set by comparing 10log10 PT to Q. If 10log10 PT (i.e. PT expressed in dB) exceeds Q, then the cell can be added, for example, to the active set. Parameter T ensures that even if the quality factors Pi of the cells currently in the active set are poor, a cell having PT (expressed in dB) less than T is never accepted as a candidate cell.
There may be other conditions, for example that the active set may not be modified too frequently, that hinder the adding of a new cell to the active set. All neighboring cells not belonging to the active set, for example, may be tested each time the neighbor list changes, and the cells whose quality factor does exceeds the acceptance criterion may be considered to be added to the active set.
The problem with the current acceptance criterion is that it does not ensure proper acceptance decisions when the value of S is different from 0 or 1. Let us consider two examples, when the following values for the parameters are set: S=2, A=−6.0 dB and T=−6 dB. In the first example, the active set contains two cells, each of which has the quality factor value 1, i.e. P1=1 and P2=1. The acceptance limit in this example is Q=0 dB, i.e. a cell having a quality factor of 1 can be, for example, added to the active set. This is an acceptable decision, because the cell has the same quality factor as the cells in the active set.
In the second example the active set contains also two cells, and now the quality factors are P1=3 and P2=3. The acceptance limit in this example is Q=9.5 dB. The quality factors of the cells in the active set are 5 dB, so a cell has to have a quality factor 4.5 dB higher than the cells in the active set to be accepted to the active set. Correspondingly, expressed in absolute values, a cell should have a quality factor PC=9.5 to be accepted to the active set. Intuitively, the acceptance criterion should also in this second example produce the result that a test cell can be accepted to the active set if the quality factor of the cell is larger than 3.
The current acceptance criterion thus leads to situation where intuitively similar situations produce a different acceptance decision. A further problem with the acceptance criterion is that for an active set whose cells have the same quality factors, from here on called an uniform active set, the acceptance limit is not equal to the quality factor.